1. Field of the Invention
The present invention relates to a multi-beam light scanning apparatus and an image forming apparatus using the same, and is suitable for an image forming apparatus using an electrophotography process, such as a laser beam printer (LBP), a digital copying machine, a multi-function printer, or the like.
2. Description of the Related Art
Conventionally, various multi-beam light scanning apparatuses are proposed, which use a multi-beam light source unit including a plurality of light sources (light emitting portions) (see Japanese Patent Application Laid-Open No. 2004-070108). In this multi-beam light scanning apparatus, if the plurality of light emitting portions are arranged in the sub scanning direction vertically, an interval (pitch) between the scanning lines in the sub scanning direction on a scanned surface is significantly larger than an interval corresponding to recording density. Therefore, the plurality of light emitting portions are usually arranged obliquely to the sub scanning direction, and a slanting angle thereof is adjusted so that the interval between the scanning lines in the sub scanning direction on the scanned surface is precisely adjusted according to the recording density.
In the image forming apparatus, the multi-beam light scanning apparatus as described above is often used to satisfy a demand for high speed. In addition, in order to satisfy a demand for a small size simultaneously, as described in Japanese Patent Application Laid-Open No. 2004-070108, a plurality of scanned surfaces (surfaces to be scanned) are scanned by a single rotational polygon mirror, and for this purpose, a structure is often used, in which, for Embodiment, a light beam enters a surface perpendicular to the rotation axis S of the rotational polygon mirror from an oblique direction with respect to the sub scanning direction. Hereinafter, this optical system is referred to as an “oblique-incidence optical system”. In the multi-beam light scanning apparatus as described above, it is important to set an imaging magnification ratio in a sub scanning section of the imaging optical system between the deflection surface of the rotational polygon mirror and the scanned surface to be uniform over the entire effective image region. It is because if the imaging magnification ratio in the sub scanning section of the imaging optical system is not uniform, a scanning line pitch of a plurality of beams on the scanned surface in the sub scanning direction does not become uniform in the effective image region. However, in the oblique-incidence optical system, when the multi-beam light source is used, if the imaging magnification ratio in the sub scanning section of the imaging optical system is set to be constant in the entire effective scanning region, the scanning line pitch of the plurality of beams on the scanned surface in the sub scanning direction becomes different between a scanning start side and a scanning end side.
FIG. 22A is a schematic diagram of a main part of the image forming apparatus using the multi-beam light source unit and the oblique-incidence optical system. The image forming apparatus illustrated in FIG. 22A includes a plurality of photosensitive members as the scanned surfaces (usually, four photosensitive members corresponding to four colors of yellow (Y), magenta (M), cyan (C), and black (K)). In this type of image forming apparatus, if scanning lines as illustrated in FIG. 22B are drawn continuously in the sub scanning direction on every scanned surface of the scanned surfaces corresponding to the individual colors, all of the four colors of Y, M, C, and K coincide. However, for Embodiment, if scanning lines as illustrated in FIG. 22B are drawn continuously in the sub scanning direction on the scanned surfaces corresponding to the colors Y and K, and if scanning lines as illustrated in FIG. 22C are drawn continuously in the sub scanning direction on the scanned surfaces corresponding to the colors M and color C, a problem will arise.
Specifically, all the four colors of Y, M, C, and K coincide at the middle portion in the main scanning direction, but Y and K are shifted from M and C in the sub scanning direction at portions other than the middle portion in the main scanning direction. As a result, all the four colors coincide at the middle portion of the output image in the main scanning direction, but at portions other than the middle portion in the main scanning direction, particularly on the scanning start side and the scanning end side, the four colors do not coincide, so that a color shift is observed. In other words, in the image forming apparatus which uses the multi-beam light source unit having the structure illustrated in FIG. 22A and uses the oblique-incidence optical system, on the scanned surfaces corresponding to colors C and M, scanning lines as illustrated in FIG. 22B are drawn continuously in the sub scanning direction. In addition, on the scanned surfaces corresponding to colors K and color Y, scanning lines as illustrated in FIG. 22C are drawn continuously in the sub scanning direction. As a result, at portions other than the middle portion in the main scanning direction, colors C and M do not coincide with colors K and Y, so that a color shift in the sub scanning direction is observed.